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L1 syndrome
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

L1 syndrome

L1 syndrome is a group of X-linked genetic conditions that primarily affect the nervous system in males, causing symptoms that range from severe hydrocephalus to spasticity and intellectual disability.

Description

L1 syndrome describes a group of conditions that primarily affect the nervous system and occur almost exclusively in males. These conditions vary in severity and include, from most severe to least, X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS), MASA syndrome, spastic paraplegia type 1, and X-linked complicated corpus callosum agenesis.

HSAS is an acronym for the characteristic features of the condition: a buildup of fluid in the brain (hydrocephalus) that is often present from before birth, muscle stiffness (spasticity), thumbs that are permanently bent toward the palms (adducted thumbs), and narrowing (stenosis) of a passageway in the brain called the aqueduct of Sylvius. In individuals with HSAS, stenosis of the aqueduct of Sylvius causes hydrocephalus by impeding the flow of cerebrospinal fluid (CSF) out of fluid-filled cavities called ventricles. Individuals with HSAS often have severe intellectual disability and may have seizures.

MASA syndrome is also named for the characteristic features of the condition, which are intellectual disability (mental retardation) that can range from mild to moderate, delayed speech (aphasia), spasticity, and adducted thumbs. Individuals with MASA syndrome may have mild enlargement of the ventricles.

Spastic paraplegia type 1 is characterized by progressive muscle stiffness (spasticity) and the development of paralysis of the limbs (paraplegia). Affected individuals also have mild to moderate intellectual disability. People with spastic paraplegia type 1 do not usually have major abnormalities in structures of the brain.

X-linked complicated corpus callosum agenesis is defined by underdevelopment (hypoplasia) or absence (agenesis) of the tissue that connects the left and right halves of the brain (the corpus callosum). People with this condition can have spastic paraplegia and mild to moderate intellectual disability.

The life expectancy of individuals with L1 syndrome varies depending on the severity of the signs and symptoms. Severely affected individuals may survive only a short time after birth, while those with mild features live into adulthood.

The conditions that make up L1 syndrome were once thought to be distinct disorders, but since they were found to share a genetic cause, they are now considered to be part of the same syndrome. Family members with L1 syndrome caused by the same mutation may have different forms of the condition.

Frequency

The prevalence of L1 syndrome overall is unknown; however, HSAS is estimated to affect 1 in 30,000 males.

Causes

Frequently Asked Questions

Management for L1 syndrome is supportive and multidisciplinary, focusing on the specific symptoms present. For hydrocephalus, surgical placement of a shunt may be necessary to drain excess fluid from the brain. Physical and occupational therapy are crucial to manage spasticity and improve motor skills. Speech therapy can help with communication difficulties. Educational support and special education services are vital for individuals with intellectual disability. Medications may be used to manage seizures or spasticity. Genetic counseling is also important for affected families.
L1 syndrome encompasses a spectrum of conditions, ranging from very severe to milder forms. X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS) is the most severe, characterized by significant hydrocephalus, severe intellectual disability, spasticity, and adducted thumbs, often present from birth. MASA syndrome is less severe, typically involving mild to moderate intellectual disability, delayed speech (aphasia), spasticity, and adducted thumbs, with milder or no hydrocephalus. Spastic paraplegia type 1 primarily features progressive muscle stiffness and paralysis of the legs, along with mild to moderate intellectual disability, usually without major brain structural abnormalities.
L1 syndrome is an X-linked genetic condition, meaning it is caused by a mutation on the X chromosome. Because males have only one X chromosome, they are almost exclusively affected and typically experience more severe symptoms. Females have two X chromosomes, so if they inherit one mutated X chromosome, they are usually carriers and do not show symptoms, or they may have very mild symptoms. A carrier mother has a 50% chance of passing the mutated gene to each son, who would then be affected, and a 50% chance of passing it to each daughter, who would then be a carrier.
Yes, L1 syndrome is diagnosed through genetic testing that identifies mutations in the L1CAM gene. This testing can be performed after birth based on clinical symptoms. Prenatal diagnosis is also possible for families with a known history of L1 syndrome. This can be done through chorionic villus sampling (CVS) or amniocentesis to test for the L1CAM gene mutation in the fetus. Genetic counseling is highly recommended for families considering prenatal testing.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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